1. Field of the Invention
The invention generally relates to a method and apparatus for determining the wafer loading status of a cassette. More particularly, the present invention relates to a method and an apparatus for determining the loading status of a cassette of a type that completely encloses the wafers, such as a Front Opening Unified Pod (FOUP).
2. Description of the Related Art
In general, wafer-mapping systems are used to determine the loading status or arrangement of wafers stored in a cassette, i.e., a wafer holder that can hold a plurality of wafers for wafer transport. The wafers are typically coaxially aligned and stacked in a spaced arrangement within the cassette. By analyzing the spatial position and/or orientation of the wafers in the cassette it is possible to detect erroneous conditions, such as a missing wafer, a broken wafer, or a cross-slotted wafer present in the cassette. Consequently, where improper loading of wafers in a cassette is detected, the processing of the wafers in such a cassette can be prevented and a possibly disastrous condition can be circumvented.
Wafer cassettes can include Front Opening Unified Pod (FOUP) systems. With FOUPs, it is known to provide a wafer mapping system on the load port interface to index along the vertical axis in order to provide presence and positional information about the wafers within a particular pod or cassette.
For example, in U.S. Pat. No. 6,013,920 Gordon et al. describes a FOUP load port interface 120 that facilitates collection of data about the number and location of semiconductor wafers within a FOUP 122, as shown in FIGS. 1a and 1b. The load port interface 120 is adapted to receive, open and close a FOUP 122. The load port interface 120 includes a bulkhead 124 that mates with and seals the semiconductor processing equipment (not shown). A vertical support brace 126 projects outward horizontally from one side of the load port interface 120 toward or away from the bulkhead 124 and supports a movable table 132. The FOUP 122 is placed on table 132 by an operator, an automatic guided vehicle (AGV) or an overhead rail system. A FOUP drive mechanism 128 rotates the FOUP horizontally so the FOUP 122 abuts with and seals against the bulkhead 124. On the other side of the bulkhead 124 from the FOUP 122, the load port interface 120 comprises a FOUP door-engaging end-effector 142 that also seals against the bulkhead 124.
Wafer sensors 186 are mounted on the end-effector 142 near its top, as shown in FIG. 1b. The wafer sensors 186 include a left-hand and a right-hand optical detector, each comprising a diode light sensor and a laser diode that emits a beam of light. A motorized door-removal drive-mechanism 162 included in the load port interface 120 supports the end-effector 142 at an upper end of a pedestal 164 (FIG. 1a). As the end-effector 142 retracts and moves the FOUP door 148 downward within cover 178, the wafer sensor 186 sequentially passes each of the wafers 4 within FOUP 122. By sensing the position of a wafer 4 as the end-effector 142 passes each of the plurality of wafers, the load port interface 120 obtains data that records both the number of wafers carried and the vertical location of the wafers within the FOUP 122.
Another wafer detection system is described in U.S. Pat. No. 6,452,503 to Weiss. That patent describes a wafer imaging system 220, as shown in FIG. 2, comprising a camera 214 or other image sensor that is positioned at a known position Zref, θ in relation to an open front of a cassette 210. The image sensor views at once an entire stack of wafers 212 in the cassette. Preferably, a light source 216 is disposed around the camera to illuminate the wafers. An image of the entire stack of wafers is captured and is image-processed to provide information on the separation of the wafers within the cassette, any cross-slotting of wafers, and the center point of each of the wafers.
A common requirement of wafer sensing systems such as those described above is that the FOUP must be opened before wafer sensing is performed. A disadvantage of such systems is that in cases where wafers are not properly loaded into the cassette, the cassette needs to be closed again and removed from the processing tool for correction of the improper wafer loading. This additional opening and closing creates unnecessary disturbances and exposes the wafers to possible contamination which preferably should be avoided. Another disadvantage is that at the input/output station, where cassettes are loaded into a processing tool, a FOUP opener mechanism needs to be present. Although many processing tools have a FOUP opener at the input/output station, this is not always the case. A system comprising a stocker, such as the A412™ furnace system of ASM, does not have a door opener available at the input/output station. In such furnace systems, after placing a FOUP cassette on the FOUP input/output station by an operator, an AGV or an overhead rail transport system, the FOUP is transferred from the input/output station to a stocker inside the system by a cassette-handling robot. When the wafers in the cassette are to be processed, the cassette is transported to a FOUP opening station, where the FOUP is opened for wafer handling. However, if wafer mapping occurs as late as just prior to the start of the wafer handling, valuable time is lost in cases where a problem with the wafers is detected. In order to avoid this time loss, wafer mapping desirably should occur when the FOUP is loaded onto the input/output station or directly after that loading.
Accordingly, there is a need for methods and systems of mapping wafers which address the problems discussed above.